University of Vermont Extension 
Department of Plant and Soil Science

Anytime News Article


Dr. Leonard Perry, Extension Professor
University of Vermont

Across the country, weather events in recent years have resulted in many flooded areas.  Hurricane Irene in August, 2011 caused epic flooding in Vermont and other northeastern states.  If you were affected, and are lucky enough to still have a home and yard, your lawn and garden may be buried under silt.  How you deal with a landscape buried under silt depends on its depth.
As with any soil, make sure it is dry before working with it.  Soil should form a ball in the hand, then crumble when pressed.  If the squeezed ball of soil drips water, it’s still too wet. 

If a lawn is covered with under an inch of silt from flooding, it may recover.  Scratch the surface with a steel hand rake or similar tool.  I have a mini-tiller with vertical blades just for penetrating compacted soil (similar to a “vertical mower”).  This will allow water and air to penetrate below a crusted surface to the roots.  Once the silt is broken up on lawns, for smaller areas you can try washing silt from the lawn, or at least thinning the depth, with a forceful garden hose.  If the silt dries and crusts, keep it broken up during the season until the grass gets reestablished. 

As soon as possible in the season, do a soil test to see what nutrients may have washed away or need replacing.  Kits are available from your local Extension service office, complete with sampling instructions.  Results will tell major nutrients needed, amounts, and if soil acidity (pH) needs correcting.

If your lawn is covered with more than an inch of silt, it may not recover and need to be reestablished from scratch.  Fine leaf fescue and perennial ryegrass have poor tolerance to submergence from flooding, especially if they were under water for 4 days or more.  If under three inches of silt, you can try renting an aerator to use up to 6 times through the season.  This removes small cores of soil and silt, allowing air and water to get to the roots.  If you do this on a smaller area, you can topdress sand or compost on top, which will work into these holes. If the silt is over 3 inches deep, consider having it professionally removed unless you have a tractor and attachment to scrape it off. 

If the silt is just too deep or your efforts are in vain, and the lawn isn’t showing signs of growth by late spring, you should probably just go over it with a rototiller incorporating the silt as if adding a layer of topsoil.  Make sure and soil test to see what needs adding before reseeding.  If a small area or you want instant results, and have the budget, consider adding strips of sod.  If this is not possible, keep in mind that the best times for seeding cool season grasses are early spring and late summer when conditions favor them and not the weeds.  If this is the case, you can stabilize the soil by seeding annual ryegrass at 4 to 6 pounds per 1000 square feet.  Then till this in late summer before seeding the permanent grasses.

Another option would be to spend this first year rebuilding the soil, removing as much silt as possible first.  Since the silt likely brought in a load of weed seeds, you’ll likely need to deal with them.  After appropriate steps as above, rather than replant, add clear plastic to “solarize” the soil.  This is basically covering the soil with a mini-greenhouse which heats up, killing some diseases and many weed seeds.  The soil could then be tilled again, bringing more seeds to the surface, and covered again.  Or, after the first covering, when uncovered seed in a cover crop.

Solarization is simple.  Best done in May or June as the soil is heating, rake the soil, moisten it with the hose if dry (moist soil holds more heat), then cover with a thick sheet of clear plastic.  Hold the edges down with boards, stones, or just bury in a shallow trench.  Leave on for 6 to 8 weeks.
If you have eroded areas to deal with, refilling and replacing the topsoil is ideal.  If this is too expensive, amend any added backfill with organic matter such as peat moss, compost, rotted wood chips or old mulch, or similar.  If the area isn’t destined to be a lawn, you can either add plantings (perennials or shrubs) and mulch, seed with annual rye, or add a cover crop such as clover.
For trees and shrubs, make sure to scoop or rake away silt deposits from the base of plants and tree trunks.  Even 6 inches deep around trunks, or 3 inches deep over roots, can be enough to smother them, resulting in a slow plant decline and perhaps eventually death.   Water and air need to reach tree roots, and since many of their feeder roots are near the surface, you’ll need to break up or remove silt as described for lawns.  Just don’t till under trees through, as this will damage their surface roots. 
Some trees that originated in floodplains survive such conditions, such as river birch and willows, but others such as pines and oaks may suffer.  Many trees are able to withstand up to a week of flooding.  Unfortunately, trees may take several years to show flood damage symptoms, and then it is too late to save them.  If you have a special tree, consult with an arborist to see if corrective measures are needed now 
As much silt as possible should be removed from vegetable gardens, and used to backfill eroded areas or perhaps make a berm garden for flowers.  Make sure to use a forceful hose to wash all such silt away from the root areas under bush fruits such as blueberries and brambles.  A general rule is that you should not harvest produce within 120 days (4 months) of flooding, as it may be adulterated from contaminants.  This does not apply to crops submerged from “ponding” or standing water such as from rain, that did not wash in from elsewhere. 
For annual flower beds, remove as much silt as possible, or till in as described for lawns. For perennial flowers, many are tough and can emerge through a few inches of soil.  Shallow-rooted ones such as yarrow and tickseed, or groundcovers such as dead-nettle and sedum, likely wont survive being buried.  If you know generally where these are, dig them in spring and replant higher or in refurbished soil.
Even though many perennials will emerge through a few inches of silt, it helps to rake over them in early spring, or around them later, to remove some silt and to keep it from crusting.  As with lawns and other beds, make sure to test the soil fertility.  If more than a few inches of silt, try to remove as much as possible in early spring before plants start growing.  Otherwise  they either wont emerge, or you’ll destroy them later in the process of silt removal.  For deeply covered perennial beds, if you can find choice plants, it may be easiest to dig them up, then till the whole bed and start over as with lawns (a chance for a new design and new plants).
If working soil that was flooded last year, or even a couple weeks prior, generally it is safe since sun, soil organisms, and rain all work to destroy harmful bacteria in sediment.  Just watch for any debris that may have washed in, or contaminants still lingering such as oil or chemicals.  Also watch for signs of the invasive Japanese knotweed that may have washed in.  If you suspect contaminants, contact your local town officials or health department, and follow precautions outlined online by the Vermont Department of Health ( 

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